The 55-year-old KSM was captured by Pakistani security forces in 2003 and transferred to Guantanamo Bay in 2006. The other four defendants were captured in Pakistan in 2002 and 2003.

The 9/11 Commission report named KSM as “the principal architect of the 9/11 attacks.”

He also has been linked to a series of other attacks by the al-Qaeda network, including the 2002 Bali nightclub bombing in Indonesia, the 1993 World Trade Center bombing and the 2002 murder of journalist Daniel Pearl.

The 2021 trial date — which will come more than 19 years after the attacks occurred — was included in a trial conduct order issued by the presiding judge in the case, Air Force Col. W. Shane Cohen, who set a series of deadlines for the case.

The order also includes a list of materials the prosecutors must provide defense lawyers before Oct. 1.

Cohen, who is the third judge since 2012 to preside over the the long-delayed case, took over the case in June.